Analysis of eligibility criteria from ClinicalTrials.gov.

Electronic health care records are being used more and more for patient documentation. This electronic data can be used for secondary purposes, for example through systems that support clinical research. Eligibility criteria have to be processable for such systems to work, but criteria published on ClinicalTrials.gov have been shown to be complex, making them challenging to re-use. We analysed the eligibility criteria on ClinicalTrials.gov using automatic methods to determine whether the criteria definition and number changed over time. From 1998 to 2012 the average number of words used to describe eligibility criteria per year increased by 46%, while the average number of lines used per year only slightly increases until 2000 and stabilizes afterwards. Whether the increase of words resulted in increased criteria complexity or whether more data elements are used to describe eligibility needs further investigation.